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Oldest animal genome is sequenced from horse bone

By Michael Marshall

Around since before the ice age

(Image: ZSSD/Minden Pictures/FLPA)

A HORSE has just taken us further back in time than ever before. The genome of a 700,000-year-old fossil has been sequenced, suggesting we could do the same with other long-extinct creatures – including early hominins like Homo erectus.

Willerslev’s collaborator Ludovic Orlando, also at the University of Copenhagen, scoured a horse bone found in the permafrost of north-west Canada in 2003 for pockets of collagen. The protein degrades easily, so its presence is a sign that DNA could have survived alongside it. Using mass spectrometry, Orlando was able to find many more collagen pockets than in past studies. He also sequenced a higher proportion of the DNA within (Nature, DOI&colon; 10.1038/nature12323).

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The team compared the ancient horse DNA to that of five domestic breeds, plus a donkey and the Przewalski’s horse – a wild species found in Mongolia. They found that the last common Equus ancestor lived between 4 and 4.5 million years ago – before the last ice age – making the lineage about twice as old as we thought.

“The whole horse history is driven by changes in climate,” says Willerslev. They thrived when the ice sheets advanced, but suffered when temperatures rose. The team is now sequencing a more recent species, which lived shortly before horses were domesticated, to figure out how that process changed its genes.

The work is a boost for efforts to recover DNA from fossils. “It’s clear that from frozen material, one can go far back in time, approaching a million years,” says Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. That would allow extinct polar animals like early mastodons and woolly rhinos to be sequenced. We could also study insects and plants to build a detailed picture of ice-age ecosystems.

Frozen material could allow extinct animals like early mastodons and woolly rhinos to be sequenced

What of our distant ancestors? Pääbo has sequenced 44,000-year-old Neanderthal remains, and an Asian group called the Denisovans that lived 30,000 to 50,000 years ago. Going further back is difficult because our ancestors did not reach the far north until relatively recently. In hot and dry climates, DNA may only last 20,000 years, and Krause says humid climates offer no hope of preservation.

Others are more optimistic about one-million-year-old genomes. “It may be possible in very deep caves, where the [DNA] is kept at very stable temperatures,” Orlando says. He says factors like the acidity of the surrounding soil could also help preserve DNA. Willerslev is bullish. “You could go back to hominins like Homo erectus,” he says, which lived between 1.3 million and 300,000 years ago.

This article appeared in print under the headline “Horse sets record for oldest genome”